A six-year-old boy from Bovingdon was killed when his mother, who was driving a black Ford Ka, lost control and crashed.
The accident happened in Ley Hill Road, Chesham, on Friday at 4.15pm.
A statement released by Thames Valley Police said: “There have not been any arrests at this time and investigations are continuing.”
No other vehicles were involved.
Anyone with information about the collision can contact Sgt Kerry Clarke or PC Julia Stroud from the road death investigation team at Bicester on 101.